1. Field of the Invention
The invention relates to sensors and, more particularly, to an optical sensor.
2. Description of the Related Art
It is known to use an optical sensor, for example, as a proximity switch, a light barrier or in a light grid, a light curtain or a laser scanner.
High demands in terms of accuracy are imposed on such a sensor. In order to meet these demands, it is conventional practice to subdivide the transmission light beam inside an optical sensor into a measuring light beam and a reference light beam using optics, i.e., prisms or a semitransparent mirror. Whereas the measuring light beam is emitted into a detection area and, if a measurement object is present, is reflected at the measurement objects as reflection light beams, the reference light beam is evaluated inside the sensor, for example, for the purpose of synchronization and calibration. For this purpose, the reference light beam is supplied to a detector with the aid of further optical elements, for example, a deflecting mirror and optical waveguides, and is accordingly evaluated using downstream components. In addition, conventional optical sensors often have transmitting and receiving optics that are separate from one another.
Consequently, it is possible to synchronize the measurement, for example, because after the reference light beam has been evaluated, the beginning and end of the emission of the transmission light beam and thus also the periods of time in which the reception of reflection light beams can be expected are fixed. If reflection light radiation is received outside these periods of time, this may only be undesirable extraneous light radiation. This also makes it possible to calibrate the sensor with respect to the insensitivity to extraneous light. Furthermore, this is particularly important when the transmission light beam from the optical sensor is pulsed or coded to avoid crosstalk from adjacent sensors. In other embodiments, the distance to a measurement object can also be determined by comparing the reference light beam with arriving reflection light beams, for example, using interference measuring methods.
In the case of an optical sensor used in safety technology, the reference light beam branched from the transmission light beam can also be used to monitor the function of the sensor. In this case, not only the functionality of the apparatus for generating the transmission light beam, for example, an LED or laser transmitting unit, but also the detector and the downstream electronics can be checked. Furthermore, the contamination of the optics of optical sensors used in an industrial environment can be monitored, in particular, with the aid of an inner reference light beam.
These known optical sensors have a wealth of disadvantages. The optical elements needed to subdivide the transmission light beam into a measuring light beam and a reference light beam and also the electrical elements needed to evaluate the reference light beam thus give rise to particular expenditure. In addition, the transmission light beam is attenuated by the beam subdivision, with the result that the apparatuses intended to generate the transmission light beam possibly must have larger dimensions to emit a sufficiently strong measuring light beam.
Another disadvantage is seen in the separate drive means for the elements for generating the transmission light beam and the elements for evaluating the reference and reflection light radiation, including the transmitting and receiving optics that are separate from one another. In order to achieve a sufficient degree of measurement accuracy, considerable measures for calibrating the individual light signals and for synchronizing the signal propagation times must be provided in such a case.